There has been an increasing demand for smaller electronic devices, especially multifunctional electronic devices. To meet this demand, embedding passive components (such as resistors, capacitors, and inductors) into the substrate has been attracting increased levels of attention. This embedding technology could provide a cost-effective way to remove passive components from the surface of the substrate, thereby providing performance, space and cost advantages. Embedded passive components have been fabricated in hybrid packaging MCM-C employing the well developed thick film technology. However, the high processing temperature (&gt;700.degree. C.) of the thick film passive components limits their use to applications which employ only ceramic substrate. Efforts have also been made to produce polymer-based printed wiring boards using MCM-L by developing polymer thick film (PTF) technology for applications in low temperature substrates. These efforts have produced polymer thick film conductors and polymer thick film resistors.
A polymer thick film resistor is a mixture of a polymer binder, a conductive material (usually in fine powder form), and a suitable organic solvent. Currently, the majority of commercially available PTF resistor pastes are in the form of thermosetting or thermoplastic resin pastes with carbon or graphite powders as conductive phase. The carbon-containing PTF resistor paste can be applied on a suitable substrate using screen printing, stencil printing or other techniques. Following the drying process, the printed pastes are cured at relatively low temperature, usually &lt;300.degree. C. The cured polymer binder provides adhesion to the substrate. It also shrinks and compresses the conductive particles together, causing electrical conduction to occur. The cured polymer binder also binds the resistor material to the substrate. The electrical conduction of the resistor is the result of the physical contacts between the particles. The resistance of the system depends on the resistance of the materials incorporated into the polymer binder, as well as their particle sizes and load. The inherent problem associated with this approach is that the resistance of the composite resistor material is highly dependent on the inter-particle contact. While the compressing force created by the polymer binder is highly dependent on the temperature and humidity conditions. As a result, this resistor system is inherently unstable and exhibits poor environmental reliability over time. In addition, these resistor compositions also suffer from poor compatibility between the copper or other conductor leads.
Thermistors are thermally sensitive resistors with very large values for temperature coefficient of resistance (TCR), which values can be positive or negative. A thermistor with a positive TCR (PTC) exhibits an increase in resistance with increasing temperature, while a thermistor with a negative TCR (NTC) exhibits a decrease in resistance with increasing temperature. Thermistors are used extensively in such applications as temperature sensors, electronic time delay elements, gas pressure sensors, voltage or current limitors, thermal conductivity detectors, liquid or gas flow sensors, solid or liquid level indicators, and the like.
PTC thermistors have conventionally been fabricated by sintering barium titanate at 1200.degree. C.-1400.degree. C. in air. NTC thermistors ordinarily consist of sintered semiconductor materials which have an NTC characteristic. Thick film thermistor pastes usually comprise powders with thermistor characteristics, a glass binder, and an organic vehicle. Thermistor elements have been produced by screen printing the thermistor paste on an insulating substrate, usually ceramic, and subsequently firing at temperatures higher than 700.degree. C. Thick film thermistors can provide large values for TCR and a wide range of resistance. However, the high processing temperature (&gt;700.degree. C.) limits them to mostly only ceramic applications.
The development of polymer thick film thermistor pastes has been limited due to the fact that high TCR materials are usually semiconductors which need to be sintered at high temperature. The highest acceptable processing temperatures of polymer thick film technology (&lt;300.degree. C.) are too low to accomplish semiconductor sintering. Alternatively, most pure metal elements exhibit large positive values for TCR. However, the low resistance of the metals and the high sensitivity of the TCR value to impurities prohibit them from being used for many applications.